THE EFFECT OF SAMPLING PROCEDURES ON ORGANIC AEROSOL MEASUREMENT

The effects of face velocity, sampling duration and filter type on apparent concentrations of atmospheric particulate organic matter have been investigated. This was accomplished by collection of ambient samples with a low volume sampling apparatus capable of collecting six simultaneous samples. Samples were analyzed by thermal-optical analysis for organic and elemental carbon.; All of the sampling parameters investigated had a significant effect on the measured concentration of organic aerosol. The apparent concentration of organic aerosol collected by quartz fiber filters decreased with increasing face velocity by 1.2 to 2.5 (mu)gC/m('3) for the face velocity range 15-150 cm/s and organic carbon concentrations of 3.1 to 15.0 (mu)g/m('3). Apparent organic carbon concentrations were an average of 19% higher for samples collected for 24 hour than for 48 hour sampling durations.; Adsorption of organic vapor by the filter medium is the most reasonable explanation for the observed differences in apparent organic carbon concentration. The adsorption artifact was estimated from carbon collected on quartz fiber back-up filters behind either quartz fiber or Teflon membrane primary aerosol filters. More than 80% of the variation in apparent concentration with face velocity was accounted for by estimates of adsorption artifact. Adsorbed vapor also accounted for a significantly higher fraction of organic mass collected, and more organic vapor was observed on quartz fiber than on simultaneously sampled Teflon membrane filters.; For samples collected on quartz fiber filters at a face velocity of 40 cm/s for a 24 hour sampling period, adsorbed vapors accounted for a lower limit of 10-15% of the collected organic carbon for a typical organic aerosol concentration of about 7 (mu)g/m('3). At lower concentrations and lower face velocities this percentage was significantly higher, and in some cases more carbonaceous vapor than aerosol was collected. A simple fluid dynamics-adsorption kinetics model is presented in order to explain the observations.; The observation of a significant adsorption artifact suggests that organic aerosol concentrations may be seriously overestimated by conventional aerosol sampling techniques, especially at low organic aerosol concentrations. Adsorption can be minimized by using high face velocities, long sampling durations and/or Teflon membrane filters in place of glass or quartz fiber filters. (Abstract shortened with permission of author.)...